Claustrophobia Chess

The idea of this variant is that a King always needs an empty and unattacked square to move to. He will panic and get a heart attack upon realising that he cannot make any move.

Rules

Same as Orthodox Chess, except for the following differences:

1. The definition of check is altered: a player is in check if their King is attacked or is not adjacent (ortho- or diagonally) to a square which is neither attacked by the opponent nor occupied by a friendly piece.
2. Black is not considered checkmate at the beginning of the game.

As usual, a player must always move so that their King is not in check after the move, and if a player cannot make a legal move and is in check, they are checkmated and lose. That means that both players are in check at the beginning, and black would be checkmate if not for the said exception.

It also means that stalemate does not exist. If a player is not in check, they have a square to move the King to.